Historically, long range gunfire correction has been achieved by the use of high powered optical devices or by spotter personnel in the vicinity of the target. Both methods are limited by such items as exposure of spotting personnel, terrain, weather, darkness, and the capability to visually acquire both the target and impacting rounds. Even under the best of conditions, range and angle estimation errors, parallax, or failure to visually spot the impacting rounds has led to the expenditure of extra ammunition in order to adjust fire. When either the gun platform or the target is in motion, the problem of correcting fire becomes even more acute.
The USAF, AC-130 gunship has, as its primary mission, the destruction of enemy supply lines. A typical mission may include an attack on moving targets such as trucks or armored vehicles. Efficient utilization of ammunition stores during a tactical mission is highly desirable and eventually will have a significant effect upon the effectiveness of military commitments by enemy forces. The AC-130 gunship employs a highly sophisticated electronic system to accomplish its mission of interdiction. The ASD-5 equipment, which is part of this system, has the capability of detecting individual motorized targets without regard to weather or darkness. The ASD-5 also has the capability of receiving and displaying S-band electromagnetic transmissions from the target vicinity and of comparing the location of any such signal with the location of the target image. A logical extension of this detection capability was the concept of an impact marker installed in the 40 mm ammunition used by the gunship. If the impact point of the round can be determined in reference to the target location, a correction may be made in aim point which should greatly increase the probability of a direct hit by the following round. Since the Low Light Level TV and Infra-Red sensors in current use aboard the gunships have only a clear weather capability of providing miss data, the electronic impact marker would allow extension of gunship operations to a true all weather capability.
Gunship equipment includes a monopulse, DF receiver integrated into the fire control system. RF signals transmitted by an electronic impact marker are displayed as a small cluster of spots on the ASD-5 screen. Position of the spot cluster is seen in relation to the target image. An angle gate is displayed on the screen as a rectangular figure enclosing the target image and is controllable as to position on the screen by the gunfire control operator. Positioning of the antenna remotely positions the pilot's sighting pipper, thus, allowing the necessary fire control correction.